A TBM of multifaceted design and capability is required for one of the most exacting tunnel projects underway in the world today. The machine developed and built by Herrenknecht AG for the Lake Mead Intake No 3 project in Nevada, USA will be asked to operate under a possible 17 bar hydrostatic pressure during excavation of the subaqueous tunnel. The TBM customer and its mechanical designer discuss the particular features required for the TBM to face the geological, logistical and risk management issues of the project.

A TBM that takes the technology to a new level of capability and operation is needed to build the new potable water intake tunnel into the base of Lake Mead, the vast reservoir created by the famous Hoover Dam on the Colorado River near Las Vegas, Nevada. The logistics of the project demand that the TBM be a hard rock machine that is able to convert rapidly from an open-faced faced system to a closed pressurised system capable of withstanding potential hydrostatic pressures up to 17 bar maximum. Water is drawn currently from two existing intake structures located on Saddle Island (Fig 1). Critical drought conditions over past seven years have lowered the surface of the lake to critical levels. If the water level continues to decline, existing Intake No 1 will become unusable. A drop in lake level to elevation 305m would render the second intake unusable.

High-specification Mixshield heading to Nevada

To address the situation the Southern Nevada Water Authority (SNWA), the project owner, has embarked on construction of a third deeper intake, Intake No 3, to solve the problem and build in extra reserve to mitigate continuing fall in lake level into the future (Fig 2).

The new tunnel project however has considerable technical challenge. The proposed configuration is complicated by several factors including the sedimentary and volcanic geology of the area, the depth of the tunnel below the mean water level of the lake with the potential for high hydrostatic water pressures at tunnel horizon, and the depth and size of the underwater intake structure at the end of the tunnel.

Because of its complexity, SNWA decided to approach the project using the design-build delivery method. After a lengthy procurement process, SNWA selected a joint venture of Impregilo S.p.A. (Milano, Italy) and SA Healy Co. (Chicago, Illinois, USA) to design and construct the new intake tunnel and structure. The joint venture, known as Vegas Tunnel Constructors, appointed Arup USA as the lead designer of the project with Brierley Associates as the geotechnical consultant. For design and manufacture of the TBM, the joint venture selected Herrenknecht AG of Schwanau, Germany.

Fig 1. Existing Lake Mead intakes

Intake access
The new intake tunnel will be accessed via a 170m deep shaft excavated by drill+blast and lined to a 9.1m final diameter by a concurrent concrete lining. Extensive probing and pre-excavation grouting is being required to advance the shaft sinking operation. At the base of the shaft, a TBM assembly chamber and starter tunnel of 137m long will be excavated. Assembling the TBM at the bottom of a deep shaft will present another major challenge. A gantry crane and strand jack system is anticipated to be used.

The intake tunnel is 4.8km long and will be supported and lined with a segmental lining of 6m inner diameter. The lining is a universal ring design and comprises five segments and a key in each 1.8m wide x 356mm thick ring. Vitally, the gasketed precast lining is designed to withstand the full, potential 17 bar hydrostatic water pressure of the full water level in the lake above the tunnel alignment.

The tunnel is located primarily in the Tertiary sedimentary rock of the Muddy Creek Formation which is divided into zones of gypsiferous mudstone, interbedded siltstone, sandstone and pebble conglomerate, tan conglomerate, and reddish brown conglomeratic breccias. At the far end of the tunnel, excavation will be through an older Tertiary conglomerate of the Red Sandstone Unit and basalt of the Callville Mesa Unit. Along the 4.8km long alignment, dozens of faults, some with major vertical displacement, will be encountered. Although many of these faults have been filled with secondary mineralization, it is presumed that the formations are recharged directly by the waters of Lake Mead and that large amounts of water will be encountered during TBM excavation.

The far end intake structure is designed as a structure that will be constructed near the shore of the lake, then towed and lowered into an underwater excavation. The structure is designed with a double bulkhead and a reception dock for the TBM.

Fig 2. Intake No 3 configuration

Dual-mode TBM
The TBM for the project is being designed as a single shield machine with capabilities to meet difficult hard rock ground conditions with some potential for high groundwater inflows and high hydrostatic pressures. The GBR (geotechnical baseline report) identifies at least two intervals that would require "pressurized face excavation, extensive pre-excavation grouting and ground treatment, or a combination thereof". To address these conditions, the TBM specification asks for a "fully shielded TBM, articulated as required, with the ability to operate in a pressurized and non-pressurized mode, and to tunnel through rock, soil and mixed face conditions". In addition, the machine should be prepared for probing and ground improvement ahead of the TBM, with 12 periphery and four face drilling portals; rapid closure within 120 seconds if operating in an non-pressurized mode; and layout for operation and access to the cutterhead chamber under up to 17 bar (247psi) of hydrostatic water pressure.

The layout of the Lake Mead TBM is based on numerous applications of high pressure Herrenknecht Mixshields and, to some extent, on the design of comparable Herrenknecht multifaceted TBM projects including the high-pressure, dual-mode, shielded rock TBM being used on the Hallandsås railway project in Sweden and the open-mode shielded rock TBMs with their extensive pre-excavation grouting capabilities and equipment that recently completed the Arrowhead water delivery tunnels in San Bernardino, California.

In order to cope with the heterogeneous ground conditions on the Lake Mead project, the TBM can be operated in open-mode as the major mode of operation with mucking out via a centre screw conveyor and continuous tunnel belt conveyor, and in semi-closed or full pressurized closed-mode to overcome limited zones of extremely difficult ground conditions and with muck transported via a fully-closed slurry circuit.

The following TBM operating modes and mining scenarios are therefore available:

• Cyclical open mode operation with consecutive pre-excavation grouting and subsequent mining cycles (Fig. 3)• Cyclical closed mode operation under reduced face pressure with consecutive pre-excavation grouting and mining cycles with open mode face access for cutterhead and disc cutter checks and maintenance• Closed mode operation under full operating face pressure and with pre-excavation grouting for open mode face access• Closed mode operation under full operating face pressure and with hyperbaric face access in air or mixed gas/saturation mode

Fig 3. TBM configuration in open mode

Open-mode operation
In open mode, buckets and muck channels in the cutterhead will feed excavated material on to the centre arranged muck hopper from where it will be extracted by a horizontally arranged screw conveyor through the ring build area. From the screw conveyor's discharge, a belt conveyor transports the excavated muck along the complete back-up section to the tunnel's continuous belt conveyor.

The muck chute layout in the cutterhead allows a first stage dewatering of the muck inside the excavation chamber. The use of a primary screw conveyor for open mode operation, and with the ability for closing the rear discharge gate, ensures the best possible and most reliable option for rapid closure of the excavation chamber in case of sudden water ingress. Although a heavy screw auger may not be the most 'elegant' solution for open-mode muck transportation, the key advantage of rapid closure is essential. In addition, the closed screw system extending through the ring-build area provides good protection of that area of activity against water spillage in wet conditions.

Fig 4. Possible drill pattern ahead of the TBM

The ability to handle water-laden muck in open-mode operation is considered to be one of the key elements. A large settlement basin is located under the screw conveyor discharge area to collect water spillage. The basin is connected to a flushing circuit that includes a small treatment plant installed to the rear of the backup gantries. This onboard circuit treatment plant configuration ensures a permanent flow of flushing water to minimize the risk of fines settling out in the spillage basins and reduces the solids content in the discharge overflow water to be pumped out of the tunnel.

Probing and pre excavation grouting
The machine is equipped with three permanently installed drill rigs (Fig 4), two of them located inside the shield for the face positions, a third located behind the ring erection area for the periphery positions, and a fourth rig capable of being temporarily installed on the erector. Probing and drilling ahead of the face can be accomplished in open mode and also in closed mode conditions for the majority of the positions using blow-out preventer units. Two identical pre-excavation grout plants are installed on the trailing gear.

Closed-mode operation
In closed-mode operation, the machine will operate in full slurry mode following the Mixshield principle with an air bubble for face pressure control (Fig 5). This system allows the machine to operate in closed conditions but with free adjustable face pressure depending on the in-situ requirements. Closed-mode operation but with reduced pressure (lower than full water head) is possible depending on the given face or ground conditions. The possibility for a water volume balance of the entire slurry circuit also allows a clear picture of water inflow quantity through the face during reduced pressure operation.

Fig 5. TBM configuration closed mode

The change of operation mode does not require modification at the cutterhead. As soon as the rear discharge gate of the screw conveyor is closed the excavation chamber is isolated and the system is closed. Before restarting in closed mode the screw casing is hydraulically retracted to clear the cutterhead center area and the slurry pumping circuit to the above-ground treatment plant is brought into operation.

The entire equipment and installation for the closed-mode slurry-pumping operation is permanently on board. The TBM is equipped with a submerged wall gate and a rock crusher in front of the suction grill and all the necessary pipework, pumps, compressed air and circuit installations in the TBM and along the trailing gear. The closed system is completed with slurry circuit installations along the tunnel, through the shaft and to the above ground treatment and slurry plant.

The tunneling system is fully prepared for hyperbaric face man-entry. A standby decompression chamber equipped with an oxygen decompression system is permanently located behind the ring build area. This chamber can be connected by an access tube to the rear shield bulkhead providing access to the drill chamber behind the front shield bulkhead (Fig 5). The size of the decompression chamber is large enough to allow for extended decompression times and to perform the complete decompression process. In addition, the system is prepared for the use of mixed gas breathing systems for higher chamber pressures. The breathing gas mixtures can be Trimix or Heliox.

For extended chamber time under high pressure, the tunneling system is prepared for a shuttle transfer of the crew between the airlock and a hyperbaric habitat at the bottom of the access shaft.

A project of such high technological demand is vital to the long term sustainability of Las Vegas. Some 90% of the city's water supply is obtained from Lake Mead which holds the waters of the Colorado River. The Colorado originates high in the Rocky Mountains and is fed primarily by the winter snow pack. It then flows through the desert of the US southwest, passing on its way through the Grand Canyon and a series of dams and reservoirs, supplying hydroelectric power and fresh water for millions of people and a variety of uses, including drinking, agriculture, and industry.

Over the last nine years, drought conditions that have prevailed in the western USA have contributed to sharp declined reservoir levels all along the Colorado. At Lake Mead, this drought has resulted in a decline in the surface level of about 35m to elevation 338m above mean sea level. To ensure the future supply of water to the Las Vegas conurbation, the Southern Nevada Water Authority (SNWA) requires that the Impregilo/SA Healy JV and its TBM supplier Herrenknecht AG succeed with one of the most demanding tunneling projects of current times.

Add your comment

Thank you for taking the time to share your thoughts and comments. You share in the wider tunnelling community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language professional.